1. Field of the Invention
The present invention relates to a piezo ceramic transformer and a circuit which drives two CCFLs(cold cathode fluorescent lamps) and more particularly to a step-up piezo ceramic transformer with two output sections and a circuit which drives two CCFLs for LCD backlight unit, working at a full wavelength vibration mode.
2. Description of the Prior Art
U.S. Pat. No. 2,974,296 discloses a piezo ceramic transformer which was first developed in early 1960s and has regained strong interest recently as a step-up transformer in inverter circuit for CCFL of LCD backlight unit.
Based on the original Rosen type design, several variations of piezo ceramic transformer, which are disclosed in U.S. Pat. Nos. 5,440,195, 5,463,266 and 5,365,141, and Japanese Patent No. 6-224484 and 8-107240, have been developed and applied successfully in inverters for LCD backlight system. Piezo transformer has many advantages compared to conventional magnetic transformer: a low profile, a narrow width, a high efficiency, less EMI, etc. In addition, unique dependence of step- up ratio on load resistance is a perfect match for ignition and regulation of CCFL, i.e. a very high step-up ratio at no load for a start of CCFL and a reduced step-up ratio for regulation of lamp current with a decreased impedance of CCFL after ignition. Most of the commercial application requires a low input voltage and thus input section of piezo transformer is multilayered to further increase step-up ratio for inverters of notebook PC, etc.
Although a single CCFL might be used as a backlight source in LCDs no bigger than 13.3", two or more CCFLs are generally required for bigger LCD than 14" to provide an increased light output and to enhance brightness, especially in LCD monitors. It is possible by increasing working frequency to increase power density of piezo transformer per unit weight, to reduce its size and to increase light efficacy of CCFL, but its working frequency is limited by characteristics of CCFL. Maximum working frequency of CCFL guaranteed by lamp manufacturer is usually no more than 80.about.90 kHz. In order to further reduce the size, especially thickness and width, of inverters for LCD, it is required to maximize working frequency within the limit and to design a piezo transformer with multiple output electrodes to drive multiple CCFLs.
Most of piezo transformers in the prior arts have only one output electrode and thus it is not possible to drive two CCFLs separately. Two CCFLs might be connected in parallel to a piezo transformer with a single output electrode, but then load impedance becomes half from parallel connection of two CCFLs, which makes it very difficult to design a piezo transformer with a matching output impedance to a load to get a high conversion efficiency.
Piezo transformer with two output electrodes 101 which is working at a half wavelength mode, .lambda./2-mode could be built with a driving section in center 103 as shown in FIG. 1, including a generating section 104 and an input electrode 102, but it has the following disadvantages: 1) a full wavelength resonance, .lambda.-mode is not working due to its geometry of input electrode and thus its working frequency is low, and 2) elemental holding is not stable because it has only a single nodal point at the center.